This invention relates to a continuous intersecting braided composite preform, a composite frame and stringer structure fabricated from such a preform, and a method of fabricating such a continuous intersecting braided preform composite structure.
Braiding is a cost effective process for producing cylindrical structures for advanced composites. For example, carbon fibers have been braided into the shapes of helicopter drive shafts, launch tubes for shoulder-launched assault weapons, and even rocket nozzles. When the fibers are subsequently resin impregnated and cured, the resultant parts are lightweight and exhibit high strengths. In addition, composites have a high resistance to most chemical and environmental threats.
Braiding flat structures is also known. Although, there are other method of fabricating composite parts including hand lay-up of layers of fabric, braiding, is a preferred method in many instances since braiding is a highly effective process in terms of material usage. This is primarily due to the fact that the plies of a structure are created in-process, without wastage. No cutting or kitting is required. And, dry xe2x80x9cbiasedxe2x80x9d fibers, i.e., those orientated at an angle, may be braided on presently available braiding machines. Biased fibers in a flat structure are desirable because they are able to handle shear loads.
In contrast, fabrication processes which begin with tape or woven products must include the step of cutting bias plies from unbiased sections causing waste. Thus, braiding provides the dual advantage of using composites in their least cost material form and doing so in a material efficient manner.
Besides being an automated process, braiding is extremely attractive because of its versatility. A standard tubular braid can be used to fabricate any mandrel configuration which can be passed through it. Braidng is excellent for producing non-symmetrical structures with complex curvatures. Finally, standard braiders can be modified to produce a wide range of components.
One problem with all composite parts, however, is that they are difficult to join together. Most attachments schemes center around those acceptable for metals, i.e. using fasteners such as rivets, bolts, clips and the like. See U.S. Pat. No. 4,109,435, especially FIG. 4.
Fastening two composite parts is especially troublesome when one continuous structure intersects another continuous structure, i.e. where stringer members intersect a frame member, where rib members intersect bar members, or where longerons intersect with bulkheads. An example is an aircraft fuselage which is made up of a thin skin supported by frame members and intersecting stringers.
Often, to attach composite stringer members to a frame member, a xe2x80x9cmouse holexe2x80x9d is cut in the frame to allow the stringer to pass through. Fasteners and additional lay up of fabric plies then secure the stringer/frame intersection. This joining process adds weight, labor, cost, and results in possible stress and fatigue points.
If the fibers of the plies of the stringer members are all non-biased, (i.e., parallel and running in the direction of and perpendicular to the stringer) and the fibers and the frame are similarly non-biased, weaving schemes to form a continuous intersection at the joint of the stringer and frame might not be too troublesome.
Constructions which include only these non-biased fibers, however, are not very strong in shear. Angled or xe2x80x9cbiasedxe2x80x9d fibers, on the other hand, in both the frame and the intersecting stringers, although exhibiting high shear strength, make it very difficult to weave a continuous intersection. So, frame members and stringer members are usually fabricated separately and then joined by cutting a hole in the frame member as described above.
It is therefore an object of this invention to provide a continuous intersecting composite preform in which the biased fibers of a stringer member are interstitially arranged with respect to the biased fibers of a frame member at the intersection thereof.
It is a further object of this invention to provide such a continuous intersecting composite preform which eliminates the need to cut the frame member or to use fasteners in order to join the frame member with intersecting stringers.
It is a further object of this invention to provide such a continuous intersecting composite preform in which the intersection of a frame and stringer is fabricated as the structure is formed.
It is a further object of this invention to provide such a continuous intersecting composite preform which eliminates the added weight, labor, material cost, and possible stress points associated with using fasteners to join such composite members.
It is a further object of this invention to provide a method of fabricating a continuous intersecting composite preform in which the biased fibers of one member are interstitially arranged with respect to the biased fibers of an intersecting member.
This invention results from the realization that the strength of a composite part at the intersection of one structural member with another structural member can be increased by interstitially arranging the biased fibers of the plies of each members at the intersection and the further realization that such a structure can be accomplished by a modified braiding machine which produces two separate flat braids, by passing the fiber packages of one braider to the carrier of the other braider to form an intersection of the braids, and then continuing to produce the two flat braids resulting in an undisturbed biased fiber preform which can be made into a continuous frame/stringer structure.
This invention features a continuous intersecting braided composite preform made up of first and second members. The first member includes at least one ply having biased fibers and the second member includes at least one ply having biased fibers. The first member intersects with the second member and the biased fibers of the first member are intersticially arranged with respect to the biased fibers of the second member at the intersection forming a continuous intersection of the first and second members.
The plies of either the first or second member or both of them can be triaxially braided including fibers biased at approximately 45xc2x0. The fibers may be carbon, ceramic, prepreg material, or a thermoplastic material. Each members could include three plies and there may be additional intersecting members each including at least one ply having biased fibers intersecting with the first member.
This invention also features a continuous intersecting structure including a resin impregnated and cured braided preform. The preform includes a frame having at least one ply of biased fibers and an intersecting stringer having at least one ply of biased fibers. The biased fibers of the frame ply are interticially arranged with respect to the biased fibers of the stringer ply at the intersection thereof forming a continuous intersection of the frame and the stringer. There may be additional stringers intersecting with the frame and each stringer includes one or more plies of biased fibers. This stringer and the frame may include a plurality of plies including biased fibers interticially arranged at the intersection of the stringer and the frame.
This invention also features a method of fabricating a continuous intersecting braided preform including the steps of producing two flat braids of biased fibers simultaneously on a modified circular braider which has two flat braiders, transferring the carrier package of each braider to the carrier of the other thereby crossing the biased fibers of each braid forming a continuous intersection of the biased fibers, and continuing to produce two flat braids on the modified circular braider.
A method of fabricating a continuous intersecting braided preform comprises producing two flat braids of biased fibers simultaneously on a flat braider; transferring the carriers of a first braider bank to a second braider bank; transferring the carriers of the second braider bank to the first braider bank; and continuing to produce two flat braids on the flat braider thereby effecting a continuous intersection of the biased, fibers between each flat braid.